1. Field of the Invention
This invention relates to a low dielectric polymer and a film, substrate and electronic part using the same. More particularly, it relates to a low dielectric polymer which has a low dielectric constant and a low dielectric dissipation factor among electric properties in a high frequency wave band, improved heat resistance up to a high temperature range, close contact or firm adherence to metal foil, and thin film formability. It also relates to a film formed of the low dielectric polymer by itself, a film formed of glass fibers impregnated with the low dielectric polymer, and substrates comprising laminated sections of such film.
2. Prior Art
In accordance with the current outstanding increase of communication information required, there is a strong demand for miniaturization, weight reduction and speed increase of communications equipment. Low dielectric electrically insulating materials are accordingly needed. For portable mobile communications and satellite communications as typified by portable handy phone and cellular phone systems, a high frequency band of mega- to giga-hertz order is used. In response to the rapid development of communications equipment used as communications means, attempts are made to tailor casings, substrates and electronic devices for small size, high density mounting. For accomplishing size and weight reduction of communications equipment adapted for operation in a high frequency band of mega- to giga-hertz order, it is necessary to develop an electrically insulating material having both satisfactory high-frequency transmission characteristics and low dielectric characteristics. More particularly, an energy loss known as a dielectric loss occurs in a device circuit during transmission process. This energy loss is undesirably dissipated within the device circuit as thermal energy. The energy loss occurs in a low frequency region due to vibration caused by a change of a bipolar electric field created by dielectric polarization while the energy loss occurs in a high frequency region due to ionic polarization and electronic polarization. The ratio of the energy consumed in a dielectric material per cycle of an alternating electric field to the energy stored therein is known as a dielectric dissipation factor, which is generally represented by tan.delta.. The dielectric loss of a material is proportional to a relative dielectric constant .epsilon. multiplied by a dielectric dissipation factor tan.delta.. Therefore, in a high frequency region, tan.delta. increases as the frequency increases. Heat release per unit area is increased by high density mounting of electronic devices. In order to reduce the dielectric loss, an insulating material with a reduced tan.delta. must be used. If a low dielectric polymer material having a low dielectric loss is used, then heat release due to dielectric loss and electric resistance is suppressed, resulting in reduced malfunction of signals. For this reason, a material having a low transmission loss or energy loss is strongly desired in the high frequency communications field. The materials proposed thus far as having such electric characteristics including electric insulation and a low dielectric constant include thermoplastic resins such as polyolefins, chlorinated vinyl chloride resins, and fluoro-resins; and thermosetting resins such as unsaturated polyester resins, polyimide resins, epoxy resins, vinyl triazine resins (BT resins), cross-linkable polyphenylene oxide and curable polyphenylene ethers.
These materials, however, have several problems when used in electronic parts or devices as low dielectric constant material. Polyolefins such as polyethylene and polypropylene as disclosed in Japanese Patent Publication (JP-B) No. 31272/1977 are improved in insulation resistance because they have a covalent bond such as carbon-to-carbon bond and are free of a bulky polar group, but inferior in heat resistance. Electric characteristics (such as dielectric loss and dielectric constant) during high-temperature operation are thus aggravated. The polyolefins are thus not suitable as an insulating film or layer in microcapacitors. Additionally, polyethylene and polypropylene are used by once forming a film therefrom, covering an electrically conductive material with the film and joining the film thereto with an adhesive. This process suffers from problems associated with film formation and coverage that working steps are complex and it is difficult to reduce the thickness of film.
Vinyl chloride resins have high insulation resistance, chemical resistance and flame retardancy, but suffer from poor heat resistance and a substantial dielectric loss like the polyolefins.
Chlorinated vinyl chloride resins have improved insulation, chemical resistance and flame retardancy, but suffer from poor heat resistance and a substantial dielectric loss like the polyolefins. Polymers containing fluorine atoms in a molecular chain such as vinylidene fluoride resins, trifluoroethylene resins, and perfluoroethylene resins are improved in electrical properties (including low dielectric constant and low dielectric loss), heat resistance and chemical stability, but suffer from several problems. These fluorinated polymers lack workability and film formability. They are substantially insoluble in commonly used organic solvents and it is thus difficult to form a thin film by solvent casting. They are also difficult to work at elevated temperature to form a configured part or film like thermoplastic resins. Manufacture of devices using these polymers adds to cost. The range of application is limited because of low transparency. Even when a fluorinated polymer can be dissolved in a special solvent to form a polymer solution and the solution be coated to a support for surface treatment, the polymer coating left after evaporation of the solvent will readily peel from the support because of the enhanced water repellent and hydrophobic properties of the polymer. It is then quite difficult to manufacture devices using these polymers. These polymers are thus applicable only in a limited range. Commonly used low dielectric polymers as mentioned above are insufficient in heat resistance since their heat resistance is classified into class B as prescribed in JIS C-4003.
Resins having relatively good heat resistance include thermosetting resins such as epoxy resins, polyphenylene ether (PPE) resins, unsaturated polyester resins, and phenol resins. The epoxy resins satisfy insulation resistance and insulation breakdown strength at heat resistant temperatures as disclosed in JP-A 192392/1994. However, the epoxy resins have a relatively high dielectric constant of at least 3 and are unsatisfactory in this respect. Another drawback of the epoxy resins is lack of thin film formability. It is known that by blending a polyphenylene oxide (PPO) resin with a polyfunctional cyanate resin and another resin and adding a radical polymerization initiator to effect preliminary reaction, there is obtained a curable modified PPO resin composition. The dielectric constant of this composition is not yet reduced to an acceptable level.
As a substitute for less heat resistant epoxy resins, a combination of a phenol-novolak resin and a vinyl triazine resin can be used. A film formed from this combination is substantially low in dynamic properties.
For the purpose of overcoming the above-mentioned problems of, for example, thermal processing, solubility in common organic solvents, and firm adhesion to metal conductors (or layers) such as copper and glass fibers such as woven and non-woven fabrics of fiber glass, there are proposed branched cyclic amorphous fluorinated polymers, copolymers of a perfluoroethylene monomer with another monomer and the like. These polymers satisfy electrical properties including dielectric constant and dielectric loss, but are less heat resistant and insufficiently soluble in organic solvents due to the influence of a methylene chain in the polymer backbone. None of fluoro-polymers available till now have good adhesion to device substrates.
Low-dielectric constant materials having improved dielectric and insulation resistance properties are further required to have sufficient heat resistance to withstand at least a heat treatment of 260.degree. C..times.120 sec. because a soldering step is always involved in a device manufacturing process. They must also be improved in chemical stability as typified by acid resistance and alkali resistance, humidity resistance, and mechanical properties. Only a limited number of polymers can satisfy these requirements. These are polyimides, polyether sulfone, polyphenylene sulfide, polysulfone, thermosetting polyphenylene ether (PPE), and polyethylene terephthalate. These polymers have film formability and adhesion to substrates, but are somewhat difficult to handle. For example, an insulating device film is prepared by dissolving the polymer in an organic solvent to form a dilute solution, spin coating the solution, and evaporating the solvent to leave an insulating film. Since those solvents which are good solvents for polyimides and polysulfones such as dimethylacetamide and N-methyl pyrrolidone are polar high-boiling solvents and have a low rate of evaporation so that some solvent may remain in the insulating film. In forming a thin film, it is difficult to control surface smoothness and uniformity. Polyphenylene ether resins and epoxy-modified polyphenylene ether resins are poor in workability and adhesion and lack reliability. Since solutions of these polymers have a relatively high viscosity, it requires skillful control to form a uniform and smooth film.